This invention relates to optical amplifiers and in particular to EDFAs (Erbiun Doped Fibre Amplifiers). It is especially, but not exclusively, concerned with L band amplifiers.
EDFAs are well known in optical transmission systems such as WDM (Wave Division Multiplex) and DWDM (Dense Wave Division Multiplex) and are used to compensate for optical losses which occur over the transmission length. Optical losses between amplification points vary according to the span distance or fibre properties as well as other factors. As a result, the input power to different amplifiers along a transmission line is not constant. As a result, amplifiers with varying optical gains have to be designed to insure constant launch power characteristics. Any fixed gain amplifier has a highly dependent gain flatness which depends on input power. Existing systems are designed using a discrete set of amplifiers each operating over a limited input power range. This is undesirable and expensive, leading to difficulties in stock control, particularly the need to manufacture and store a variety of different amplifiers.
Document U.S. Pat. No. 6,222,670 shows a multistage optical amplifier in which the length of a fibre of a second EDFA was charged as part of the procedure of analysing the performance of the amplifier.
An alternative approach is to design variable gain amplifiers which have a variable optical attenuator (VOA)to engineer the inversion required along the length of the amplifier. Such amplifiers allow around 8–10 dB of dynamic input power range without significantly impairing the optical signal to noise ratio (OSNR). However, they require the amplifier to be designed for the highest gain they will ever need. This maKes them expensive. VOA control is complex and adds in the order of 20% of the cost compared to already expensive fixed gain amplifiers.
Thus, there is a problem in providing relatively inexpensive amplification in environments where a number of different amplifiers are required having different input powers. The present invention addresses that problem.